The class XmlContent is a kind of replacement for Read and Show:
it provides conversions between a generic XML tree representation
and your own more specialised typeful Haskell data trees.

If you are starting with an XML DTD, use HaXml's tool DtdToHaskell
to generate both the Haskell types and the corresponding instances.

If you are starting with a set of Haskell datatypes, use DrIFT to
derive instances of this class for you:
http://repetae.net/john/computer/haskell/DrIFT
and _do_not_ use the current module, but rather
Text.XML.HaXml.XmlContent.Haskell, for the correct matching
instances for standard Haskell datatypes.